1. Field of the Invention
The present invention relates to a circuit for operating a gaseous discharge lamp from the AC means as well as from an auxiliary DC source, and more particularly, to a circuit for operating such a lamp under normal conditions at power frequency from a AC line source of electrical energy in conjunction with a reactor ballast, and, under emergency conditions, at high frequency from an inverter powered by an auxiliary DC electrical energy source.
II. Description of the Prior Art
A power failure, no matter what the cause may be, may very well jeopardize human life due to lighting system failure. There are therefore, many installations which require some type of emergency lighting system which will automatically come into operation upon the occurrence of a power failure; the high efficiency of a fluorescent lamp makes it especially valuable for use in such a system.
Presently available emergency lighting systems are generally of the type using a transistor switching inverter and wherein a single fluorescent lamp, or group of lamps, is used both for normal AC operation of the lighting system and for the emergency system, a battery being used as the power source for energizing the transistor inverter and the lamp, or lamps, upon loss of AC line voltage. Ideally, such an inverter is of the high efficiency type and is provided with means for controlling its operation; such a system is disclosed and claimed in U.S. Pat. No. 3,921,005 -- Watrous, assigned to the assignee of the present invention.
When attempts were made to apply an emergency lighting system, such as for example that described in U.S. Pat. No. 3,906,243 -- Herzog, assigned to the assignee of the present invention, to fluorescent lamps in the size range of 20 watts and less for operation in the rapid start mode, the isolated secondary winding of the ballasts was found to have too low an impedance at high frequency. Designing this winding with a sufficiently high impedance makes it large, with too high an open circuit voltage (causes rapid start lamps to instant start) and hence lossy.
Examining other ways of starting fluorescent lamps suggests the use of a manual starter, a gas-filled bi-metal starter or a solid-state equivalent. While the manual starter is compatible with a high frequency emergency inverter, where emergency lighting is needed, a manual starter is not desirable nor suitable. Both the gas-filled bi-metal starter and the solid-state equivalent will act to short out the high frequency inverter rather than start the fluorescent lamp in the emergency mode. This means that, if the fluorescent lamp could be started with the reactor, but without the normally employed starting methods, an emergency/normal lighting system could be developed for 20 watt or lower wattage lamps and as well as for the overseas market where, with the 220 V AC supply, reactor ballasts are employed for lamps up to the 65 watt level.
It is desirable, therefore, to provide a lighting system wherein a high frequency inverter is compatible with a reactor ballast for operating a gaseous discharge lamp both from the AC mains, and upon the failure thereof, from an auxiliary DC source.
Accordingly, it is an object of the present invention to provide a lighting system including a circuit having a reactor for operating a gaseous discharge lamp during normal conditions from the AC mains, and having a controlled transistor switching inverter for starting the lamp during normal conditions and for starting and operating the lamp during emergency conditions: ie, when the AC line voltage has dropped below a predetermined level.